Second-generation mobile communication refers to the transmission and reception of voice in a digital form, such as CDMA, GSM, and the likes. GPRS, which is more advanced than the GSM, has been proposed, and the GPRS is a technology to provide a packet switched data service based upon the GSM system.
Third-generation mobile communication refers to the transmission and reception of video and data as well as voice, and 3GPP (Third Generation Partnership Project) developed a mobile communication system (IMT-2000) technology, and adopted WCDMA as a radio access technology (hereinafter, referred to as “RAT”). By combining both of such an IMT-2000 technology and a radio access technology (RAT), e.g., WCDMA, it is called UMTS (Universal Mobile Telecommunication System) in Europe. Furthermore, the term UTRAN is an abbreviation of UMTS Terrestrial Radio Access Network.
Fourth-generation mobile communication is the fourth generation of wireless mobile communications standards. It is a successor of 2G and 3G families of standards. A 4G system is expected to provide a comprehensive and secure all-IP based solution where facilities such as ultra-broadband (giga-bit speed such as 1000+ MiB/s) Internet access, IP telephony, gaming services, and streamed multimedia may be provided to users. LTE (Long Term Evolution) is one of the 4th generation wireless standards designed to increase the capacity and speed of mobile telephone networks. E-UTRAN is the air interface of an LTE upgrade path for mobile networks. It is the abbreviation for evolved UMTS Terrestrial Radio Access Network.
In LTE, when the ciphering function is activated, transmitted data, which is ciphered using a COUNT value generated by a transmitting side PDCP, is deciphered using a COUNT value expected by a receiving side PDCP. According to 3GPP LTE PDCP spec (TS36, 323), a COUNT value includes a PDCP SN and an HFN, and the receiving side generates a COUNT value using a received PDCP SN and an expected HFN value. However, if the channel status is not good, a state of not receiving sufficient grants from an eNB may continue, and this may cause the discarding of a large amount of PDCP SDUs from the transmitting side when the Discard Timer is activated. In the case of using an ciphering algorithm other than Null, if 2048 or more PDCP SDUs with sequence numbers (SNs) assigned thereto are discarded, the HFN value used by the transmitting side and the HFN value used by the receiving side are different from each other, which is an invalid deciphering result, and therefore data loss may occur at the application end. At present, any method for preventing or restoring this is not mentioned in the 3GPP LTE PDCP spec.